Thanks to everyone for help with this problem with large spinning sidebands
that wouldn't shim away.
Our problem was found to be an unresponsive non-spinning shim (Z2Y). The
first clue was that changing this shim value did not change the lock value
appreciably, unlike the other non-spinning shims.
We also ruled out a problem with the cryoshims by comparing the spectrum
with the RT-shims all set to zero and the spectrum with the RT-shims
unplugged from the power supply. The idea being that if both spectra had
been the same, a problem with the cryoshims would have been suspected.
However, the zero shims spectrum showed terrible sidebands (no center peak,
but lots of sidebands) and the unplugged shims spectrum did not.
I replace the DAC chip for this shim, which was actually looked burnt, and
the problem appears to be solved. From now on, I will keep a couple of
spare chips on hand. :)
FYI, other possible causes of the sidebands that were suggested were:
- Damage to shim wiring from condensed water
- Ferromagnetic material in the probe or magnet bore
- Try cleaning the magnet bore and inspect the shim stacks
- If spinning sidebands are not in phase with the signals (unlike real SSB),
check for problems with probe, such as a broken glass insert in the probe or
problems like wobbly spinning, coils that are not perfectly aligned with the
spin axis of the sample, or a tube that is not straight.
- RF-inhomogeneity or acoustical signatures that can look like SSB
Thanks again,
Stephanie Mabry
-----Original Message-----
> From: Stephanie Mabry [mailto:mabry_at_umbc.edu]
> Sent: Monday, April 22, 2002 2:35 PM
> To: AMMRL
> Subject: Reoccurring spinning sidebands
We have a QE-300 with a Tecmag pulse programmer that is not scheduled to be
replaced for another 2 1/2 years. (We have the money, but are waiting on the
space to be constructed.) The 1H-13C probe is a reconditioned probe just
bought in January.
Over the past couple months, we have had a problem with very predominant
spinning sidebands. Initial attempts at shimming them away didn't work, but
at one point, I was able to reduce the first order sidebands to below 2%.
This lasted two weeks and now we have very large first, second, third,
etc. -order sidebands. Adjusting the non-spinning shims currently only
reduces the sidebands to around 6-7%.
Any ideas what is causing these spinning sidebands to return? I was
wondering if the magnet bore could need cleaned. The lineshape looks good.
Thanks for the suggestions.
Stephanie Mabry
--------------------------------------------
Stephanie Mabry, Ph.D., Instrument Manager
University of Maryland, Baltimore County
Dept. of Chemistry and Biochemistry
1000 Hilltop Circle
Baltimore, MD 21250
office (410)455-1031
fax (410)455-2608
mabry_at_umbc.edu
Received on Fri May 03 2002 - 11:09:54 MST